There are several common techniques for power over Ethernet (“PoE”) that have been developed and used in practice. The Institute of Electrical and Electronics Engineers (“IEEE”) established and continue to establish various standards for PoE, namely, IEEE 802.3 and more specifically 802.3af, 802.3at, 802.3bt, etc. The IEEE standards provide for signaling between the power sourcing equipment (“PSE”) and powered device (“PD”).
PSE is a device such as a network switch that provides (or sources) power in common mode over two or more of the differential pairs of wires found in the Ethernet cable. A PD is a device powered by a PSE and thus consumes energy. Examples include wireless access points, Internet Protocol (“IP”) phones and cameras, wireless access points, etc.
An energized PoE electrical path is not “closed” or “made,” electrically coupled or in electrical communication when the PD contacts physically engage the PSE contacts (i.e., power does not pass from the PSE to the PD, or vice versa, simply by engagement of the respective contacts), rather the standards provide a protocol with stages of powering up an energized PoE electrical path. Control circuitry associated with the PSE functions in accordance with certain instructions to perform a series of steps. First, the PSE detects the classification resistance of the PD. Second, the PSE outputs an initial classification voltage and reads the load at the PD to confirm correct classification of PoE. Third, the PSE outputs a ramping startup voltage so that current will begin to flow. Fourth, the PSE supplies a normal operating voltage and current flow to the PD.
The maximum continuous output power a PSE can sink per Ethernet cable was originally the 802.3af PoE standard with ˜13 W that would be available at the PD input's RJ-45. Since then, the market has continued to demand more power. So, in 2009, the IEEE standard was revised and released IEEE 802.3at (also known as PoE+), which increased the maximum PD power level to 25.5 W. Currently, the IEEE 802.3bt (also known as PoE++ or 4PPoE), will provide PDs with up to 71 W of power (Type 3) or up to 90-100 W (Type 4), where each twisted pair will need to handle a current of up to 600 mA (Type 3) or 960 mA (Type 4). With more power, developers can easily add more features and upgrade existing products. It is conceivable that the current maximum PSE power outputs will continue to rise (for example, 60V at 2 A (120 W) has been proposed) as further developments are made related to PoE.